It's a mix I made.
I make Beechem layout fluids for machinists.
So I made up a modified mix..little more dye..and thinned it out a bit.
Doesn't stink like a marker either.
I just paint it on with a tiny artist's brush.
It's thin enough that it runs a bit up and down the barbs..kinda gives it a neat look...if I do it right.
Those above were just me trying out a different pattern.
I normally do one like this....
Seems to work pretty good.
Stays on very well.
But...it doesn't like to go onto the underside of the feather very well.